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Mineral equilibria and formation of diamond in the mantle eclogites

Butvina V.G., Litvin Yu.A.*

(Moscow State University, geological department, Moscow, Russia)

*(Institute of experimental mineralogy RAS, Chernogolovka, Russia)

butvina@hotbox.ru; phone: (095) 9392971

Phase relations of multicomponent system garnetclinopyroxene (omphacite) are

investigated at 7 GPa and 12001600, conditions of diamond stability.

The experimental data combined with published data for boundary components permit to construct a liquidus surface for the ternary omphacite MgCaGrt FeCaGrt system and to definite physicalchemical peculiarities of diamondbearing eclogite formation. According to modern experimental [1] and mineralogical [2] researches there are two natural parent mediums: sulfide and carbonatesilicate, which take part in diamond formation. According to the experiments silicate melts cant be effective solvent for carbon, but its necessary for spontaneous diamond nucleation.

Mineral composition of Diamondbearing Ilmeniterutile MgFe eclogite (group 3) was taken from numerous types of eclogite (26 genetic groups of garnet after claster analyze [3]) for our experiment. The compositions were calculated as the most representative for the Maksutov Complex (Southern Urals) and reflect some key peculiarities of eclogite minerals from the mantle nodules in kimberlite pipes (table 1).

Table 1. Chemical analyzes of garnets and clinopyroxenes from mantle nodules in kimberlite pipes Ruslovaya and Obnazhennaya.* Mineral Component, mol.% Name of pipe SiO TiO Al2O FeO + Fe2O MnO MgO CaO Na2O 4/ Grt*** 37. 0. 21. 25. 0. 4. 11. 0. 4/ Cpx 54. 0. 8. 7. 0. 8. 14. 6. Grt 38. 0. 21. 27. 0. 3. 8. 0. Obnazhennaya Cpx 54. 0. 7. 9. 0. 7. 15. 5. RSL Grt 37. 0. 21. 28. 0. 3. 8. 0. Ruslovaya RSL Cpx 55. 0. 7. 8. 0. 8. 14. 4. Claster group 3** Grt 38.30? 42. 0.00 ? 0. 19.52? 23. 11.59 ? 28. 0.17 ? 0. 7.50 ? 17. 2.50 ? 14. 0.00 ? 0. Different pipes, total 129 analyzes Claster group 4** Cpx 51.37 ? 56. 0.00 ? 1. 3.43 ? 12. 1.56 ? 9. 0.00 ? 0. 7.92 ? 17. 10.00 ? 20. 2.81 ? 7. Different pipes, total 176 analyzes * Data from [9].

**Data from [3]. Claster group 4 of clinopyroxene from: Mg, MgFe, Al diamondbearing eclogites.

***Used symbols: Grt garnet, Cpx clinopyroxene, Alm almandine, Prp pyrope, Grs grossular, Jd jadeite, Hd hedenbergite, Di diopside, L melt.

Previous experiments in the eclogitic system [4] gave a possibility to specify the phase fields in the pseudobinary system garnetclinopyroxene, as well as to define the nature of the garnetclinopyroxene equilibrium. The present experiment was performed using PtRh and Pt ampoules, which were preliminarily equipped with tungsten foil of 0.02 mm in thickness. This foil was applied in order to prevent from the partial absorption of iron from a melt by the PtRhalloy. As a result, during the partial oxidation of tungsten the buffer couple W/WO2 was formed, which conditioned the stability of ferrous iron [5].

Pressure and temperature were regulated with an accuracy of 0.1 GP and 10, respectively, in the range of 1500 1700, and 20 at higher temperature. After runs samples were studied by electron microprobe CamScan equipped with EDS Link AN 10/85S (Department of Petrology, analysts E.V. Guseva and N.N. Korotaeva) and WDS Camebax (IEM RAS, analyst A.N. Nekrasov).

Run conditions and their results are shown in table 2. A phase diargam for the clinopyroxenegarnet system at 7 GPa is demonstrated in fig. 1. The garnet omphacite join is an internal polythernmal cross section of the system (almandine + grossular) (pyrope + grossular) clinopyroxene (fig. 4), whose liquidus equilibria are very important for petrolgical constraints.

Table 2. Conditions and results of experiments in the pseudobinary system garnetclinopyroxene at 7,0 GPa.

# GrtCpx Ampoule t, min components, mol.% T, C phases SiO Al2O FeO MnO MgO CaO Na2O begin.comp.

Grt 37, 21, 25, 0, 4, 11, Cpx 54, 8, 7, 0, 8, 14, 6, PtRh e..* Cpx+Grt Cpx 56, 12, 9, 0, 4, 8, 8, Cpx 55, 12, 7, 0, 6, 11, 7, Grt 38, 20, 22, 0, 6, 11, Grt 39, 20, 24, 0, 4, 10, PtRh e.. Cpx+Grt Cpx 54, 16, 4, 0, 6, 11, 5, Grt 40, 22, 12, 0, 10, 12, PtRh e.. Cpx+Grt Cpx 55, 11, 10, 0, 5, 10, 7, Cpx 55, 12, 8, 0, 5, 11, 6, Cpx 55, 11, 7, 0, 6, 11, 7, Grt 40, 19, 19, 0, 8, 11, Grt 38, 20, 23, 0, 6, 11, PtRh, W e.. Cpx+Grt+L L 50, 15, 13, 0, 6, 10, 2, Grt 40, 21, 20, 0, 6, 9, Grt 41, 22, 18, 0, 8, 9, Cpx 55, 14, 6, 0, 6, 10, 7, Cpx 55, 15, 5, 0, 6, 9, 7, PtRh, W e.. Cpx+Grt+L L(calc.)** 47, 15, 16, 0, 6, 13, 3, Grt 40, 21, 23, 0, 5, 8, Grt 39, 21, 23, 0, 6, 9, Grt 39, 21, 22, 0, 7, 9, Cpx 55, 11, 7, 0, 6, 11, 6, Cpx 55, 10, 7, 0, 8, 12, 6, PtRh, W e.. L L(calc.) 47, 15, 16, 0, 6, 13, 3, L 41, 12, 14, 0, 10, 14, 7, L 45, 14, 17, 0, 5, 14, 2, L 50, 14, 12, 0, 4, 16, 1, *e.. assosiacion of equilibria in the definite field of phase diagram;

**composition of melt calculated for cotectic temperature (1500).

Firstly, consider the phase relations in the pseudobinary system omphacite garnet (fig. 1). Its subsolidus involves two fields: (1) Cpxss and (2) Cpxss + Grtss. Cinopyroxene is a solid solution of diopside, hedenbergite, jadeite and CaEskola and low CaTschermak molecules. A boundary between monophase and binary fields is a curve of thermal dependence for composition of clinopyroxene solid solution. Melting of the binary assemblage omphacite + garnet starts at 1400 and is eutectic. A presence of the Cpx+Grt+L field corresponds to the monovariant cotectic in the ternary system. As temperature is drown higher, compositions of all cotectic phases change. In samples corresponding to the cotectic region, glass was homogeneous, without dendritic textures. Glasses in the samples 910 and 916 were not analyzed, whereas their compositions were estimated from the bulk composition in the point of complete melting at 1500 (Cpx50Grt50, mol. %), that is the minimum temperature of liquidus.

Clinopyroxene and garnet are liquidus phases. As a rule, melts were homogeneously quenched. Locally, dendritelike quenching textures form (fig.

2). Fig.3 shows a sample of ternary cotectic assemblage Cpx+Grt+L, whose compositions are presented in Table 2. Slight increase of pyrope component in garnet occurs with the increasing of temperature in subsolidus and the decrease of temperature in liquidus.

Fig.1. Diagram of pseudobinary system on clinopyroxenegarnet join at 7,0 GPa.

Fig 2. Dendritic quenching textures of phases. (Cpx50Grt50, # 723, 7 , , 30 min).

Fig.3. Equilibrium cotectic assemblege clinopyroxene+garnet+L (# 908, 7 GPa, 1450 o, 50 min).

Fig.4. Liquidus surfice of the ternary eclogitic system clinopyroxenegarnet at 7 GPa; studied pseudobinary join (fig.1) is shown by thick dashed line; figures at isotherms (thin dashed lines) correspond to temperatures in .

Fig. 4 shows the liquidus surface of pseudoternary system (almandine + grossular) (pyrope + grossular) omphacite at 7.0 GPa, that is constructed using the above experimental data for the pseudobinary join omphacite garnet (dasheddotted line in fig. 4). Temperatures of melting of the boundary phases was estimated using the known data on endmembers of the Cpxss Grtss solid solutions. The system almandine pyrope is studied previously at 6.5 GPa [6].

It is characterized by wide solubility of components both in solids and liquids. Low concentrations of grossular do not significantly change this pattern, since grossular is an endmember of the garnet solid solution. The system almandine(grossular) clinopyroxene. Temperature 1450 at Cpx40Alm of the eutectic is estimated from special runs in the system almandineclinopyroxene. These data allowed reconstruction of the monovariant cotectic in the ternary system. The join pyropeclinopyroxene is the third boundary system. The temperature (1520 ) and composition of eutectic in this join were evaluated from published data on diopside, pyrope, and jadeite [7,8].

According to the ternary diagram there is no reason for the formation of inverse Mg/Fe zonation in garnets at constant pressure. Since the composition of garnet is controlled by the boundary pyropealmandine join, the thermal evolution can result in direct zonation only, i.e. Mgnumber of garnet must decrease toward rims of individual grains. The inverse zonation, shown in [6], could appear as a result of abrupt decompression.

Problem of diamond formation in eclogites cannot be solved in silicatecarbon system only. Preliminary experiments on eclogitecarbonatitesulfidecarbon system at 68 GPa established immiscibility of silicate and sulfide melts in UHP environments and demonstrated leading role of carbonatitesilicatecarbon and sulfidecarbon melts in diamond formation in the Earths mantle.

Study is supported by project Scientific School 1301.2003.5, RFBR (projects 010564598, 020564684, 030506288) and priority project of RAS 10b, 2003.

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